Polishing stainless iron and steel



Patented June 26, 1945 POLISHING STAINLESS IRON AND STEEL Irvine C.Clingan, Baltimore, Md., assignor to Rustless Iron and SteelCorporation, Baltimore, Md., a corporation of Delaware N Drawing.Application February 12, 1941,

Serial No. 378,576

2 Claims. (Cl. 204140) The present application is a continuation in partof my co-pending application, Serial No. 319,056, filed February 15,1940.

The present invention relates generally to the art of manufacturingstainless iron and steel, and more particularly to the finishing ofstainless iron and steel products such as wire, rods, plates, sheets,strip, rounds, bars and the like, and the articles fabricated therefrom.

Stainless iron and steel products have a surface which has acharacteristic grayish appearance. This grayish appearance is possessedby such products even though subjected to cold rolling or drawing tosize during the manufacture thereof, either with or without intermediateor final annealing. Furthermore, where articles are fabricated from suchproducts by further rolling, drawing or forging, they still possess thisgrayish appearance. In most cases, this grayish appearance is quiteobjectionable and it has been necessary to resort to various means forproviding a polished surface.

Prior to the advent of electrolytic polishing of stainless iron andsteel products within the past few years, the only available method ofobtaining a satisfactory high, mirror-like finish was by mechanicalpolishing. The mechanical methods of polishing have been foundunsatisfactory in view of the high cost thereof and in view of the factthat considerable material is wasted due to the lack of care in thecarrying out of the bumng and grinding operations. As a consequence, ithas been proposed to polish stainless iron and steel products andarticles fabricated therefrom by anodic treatment in various types ofelectrolytic baths.

It has been proposed heretofore to polish iron and steel products byanodic treatment in an electrolytic bath containing sulphuric acid. Suchattempts have been unsuccessful for various reasons. In such priorattempts to utilize an electrolyte containing sulphuric acid, the acidconcentration, temperature, current density and time of treatment havenot been properly correlated in order to give a satisfactory bright,mirror-like polish to the material. I have found that careful controlmust be exercised over these various factors where an electrolytecontaining sulphuric acid is used or the stainless iron or steel will beadversely rather than beneficially affected by the treatment. Inaddition, in some of the prior attempts to use sulphuric acid as theprincipal constituent of the electrolyte, other acids have been addedwhich adversely affected the character of the material treated. Forexample, it has been proposed to use a mixture of sulphuric acid andhydrofluoric acid for the olishing of stainless steels. This bath isobjectionable because of the fact that pitting of the material resultsif inappropriate current densities for polishing are used. Another ofthe difliculties which has been encountered in the prior attempts to usesulphuric acid as the principal constituent of the electrolyte is thatits field of usefulness is limited. Prior attempts have been made .touse such an electrolyte for the polishing of various types of iron andsteel other than chrome-nickel steels, but these attempts must havefailed because I have found that it is impossible to obtain a polishexcept in the case of stainless steels of the chrome-nickel types. Ihave found that even the straight chrome stainless steels can not besatisfactorily polished with thi electrolyte.

In accordance with my invention I have found that the chrome-nickelstainless iron and steel products can be given a high mirror-like polishby anodic treatment in a bath consisting essentially of sulphuric acidand water. As stated above however, polishing can only be obtained wherethere is proper correlation of the acid concentration of the bath, thetemperature of the bath, the current density employed and the time oftreatment.

In carrying out my invention the electrolyte should contain fromapproximately to sulphuric acid, the balance being water. Where the acidconcentration falls below approximately 60% a sufiicient polish cannotbe obtainedeven in the case of the chrome-nickel stainless steels.

The appropriate operating temperature will vary somewhat with the acidconcentration of the bath. I have found that where the acidconcentration is between approximately 60% and 90%, satisfactorypolishing can be obtained where the temperature of the electrolyte isbetween approximately 30 C. as a minimum and approximately 50 C. as amaximum. Of course some small amount of polishing can be obtainedslightly beyond the temperature limitations just mentioned where slightvariations in the acid concentration and the other factors involved aremade. However, I have found that proper polishing withinreasonable timelimits and without pitting cannot be obtained where there is anysubstantial variation from the operating ranges set fort herein.

The current density required in order to obtain the desired high,mirror-like finish will vary somewhat in accordance with the acidconcentration and the temperature of the electrolyte.

' rent density slightly less than 6 amperes per square inch is employed,but in no case should a current density of less than approximately 3 to4 amperes per square inch be used. Attempts to use low current densitiesresult in a gray etching of the material.

The time required inorder to obtain a satisfactory polish'will vary'inaccordance with the current densities, temperatures and acidconcentrations used. I have found that under the conditions mentionedabove, the time required to obtain a satisfactory polish will vary from1 to 5 minutes. Ordinarily it will require approximate- 1y 3 to 5minutes in order to obtain the desired polish.

In carrying out my nickel stainless iron or steel to be treated isimmersed in the electrolyte and direct current of appropriate densitypassed through the material and then through the bath to the cathode.The steel being treated is the anode in the circuit and it may besupported in the bath in one position. or, if the article is of suchcharacter as to lend itself to a continuous operation, it may be passedcontinuously through the bath.

The tank and cathode will be selected in order to suit the bathrequirements. The tank may be lead-lined or formed of some other metalwhich will resist the attack of the acid electrolyte. The cathode may bein the form of a plurality of lead plates of suitable size andconfiguration. The cathode area does not appear to be critical, but inorder to obtain proper results, it is desirable to have the cathode of asize and configuration closely approximating that of the anode.

One of the particular advantages of my invention is that the acidemployed is relatively cheap and non-volatile at the preferred operatingtemperatures. Another advantage is,v as indicated above, that lead-linedtanks may be employed. The use of a lead-lined tank is advantageous inthat the equipment employed for polishing can be modified without anyparticular trouble or expense for use as an electroplating tank forchrome or nickel plating. This can be accominvention the chrome iron andsteel products pickled in this way has not been obtained. Even after thepickling of such material it possesses a grayish or whitish appearancewhich renders the material undesirable for certain purposes, and where ahigh mirrorlike polish has been desired, mechanical polishing or buiiinghas been utilized. As stated above, I am also aware of the fact thatattempts have I been made to pickle and polish stainless iron and turesthe hydrofluoric acid is rapidly lost. Where sulphuric acid alone isused no essential change takes place in the electrolyte during thecontinuation of the polishing process, except that it must bereplenished periodically by the addition of water and sulphuric acid. Asa consequence, where my invention is employed, it is relatively easy tomaintain uniform operating conditions over a long period of time.

While I have described the ranges of temperatures, current densities andacid concentrations which may be employed in the polishing. ofchrome-nickel stainless iron and steel, it should be understood that myinvention is not limited to these specific conditions, as-somesatisfactory polishing-may be obtained slightly beyond these expressedlimitations without any departure from my invention. My invention may beDracticed otherwise than as specified above within the scope of thefollowing claims.

I claim:

1. The method of. polishing chrome-nickel stainless iron and steel byanodic treatment comprising immersing the metal to be treated in a bathcontaining 60% to 90% by weight of concentrated sulphuric acid and theremainder water, and passing direct current through the bath of adensity of 6 to 12 amperes per square inch plished merely by reversingthe polarity of the electrodes and by adjusting the distances betweenthem. Sulphuric acid is the electrolyte commonly employed in suchelectroplating operations.

I am fully aware of the fact that a sulphuric acid bath has been usedheretofore in the pickling of stainless iron and steel of thechromenickel variety for the removal of scale therefrom. Sulphuric acidbaths have been used for both electrolytic and straight dip pickling ofstainless iron and steel. However, polishing of stainless of the treatedmetal surface for such period of time as to prevent gray etching of thetreated metal and to polish it while using the metal as the anode andwhile maintaining the bath at a temperature of approximately 30 C. to 50C.

. 2. The method of polishing chrome-nickel stainless iron and steel byanodic treatment comprising immersing the metal to be treated in a bathconsisting essentially of approximately 60% to by weight of sulphuricacid and water, and passing direct current through the bath of a densityof approximately 6 to 12 amperes per square inch of the treated metalfor a period of approximately 3 to 5 minutes to give a mirrorlike polishto the metal while at the same time preventing a gray etching of thesurface, all while using the metal as the anode and while maintainingthe bath at a temperature of approximately 30" C. to 50 C.

IRVINE C. CLINGAN.

